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Full text of "Handbook Of Chemical Engineering - I"

THE TRANSPORTATION OF GASES
157
The angle of convergence or taper of the inlet cone is best taken at about 15 deg. to the axis (under which condition the coefficient of influx is nearly unity), and should not exceed 30 deg.   Its larger diameter should be at least 25 per cent greater than the small diameter (see also Fig. 12, full lines). Where  a fan  draws its  air  direct from the atmosphere or from the room in which it is placed and does not have a pipe attached to its inlet, it is of material advantage to use a fan with two inlets.    For a given volume delivered the inlet velocity where two inlets are used will   be   one-half   (and   therefore   the   inlet velocity pressure  one-quarter) that with   one                 Inlets of fan casings.
inlet.    The effect  of  this reduction in inlet
losses is illustrated in the table below, where the two fans are identical except as to inlets.
FIG. 7
f
FIG.  10.—Diffuser casing.
FIG. 11.
Inlet	Revolutions per minute	Static pressure, inches of water	Cubic feet per minute	Air horsepower	Brake horsepower	Mechanical efficiency, per cent
Single ......	2G5	1 5	39 , 200	12.50	25 0	50 0
Double	265	1 5	44 100	14 95	27 7	54 0
						
Another test, in which the volume and static pressure were held constant instead of the revolutions per minute and static pressure, shows the decrease in revolutions per minute and increase in mechanical efficiency of the double-inlet fan, as follows [see also Figs. 13 (full lines) and 12]:
Inlet	Revolutions per minute	Static pressure, inches of water	Cubic feet per minute	Air horsepower	Brake horsepower	Mechanical efficiency, per cent
Single      ......	265	1.5	39 , 200	12.50	25.0	50.0
Double ..... ...	253	1.5	39,200	12. 50	22.6	55. 2
						
This gain in volumetric and mechanical efficiency holds only with a single-width fan. A double-width, double-inlet fan is in effect simply two single-width, single-inlet fans placed back to back. Obstructions to the flow of air into the inlets, such as bearings and bearing supports, are often important offenders. The inlet area is